16.1 Introduction to the Reproductive System

Introduction

Reproduction is the process of forming a new organism. In humans, reproduction is carried out by the male and female reproductive systems. Without the process of reproduction, humans (and all living organisms) would cease to exist.

The human is considered a diploid organism because most of its body (somatic) cells contain 23 pairs, or two complete sets, of chromosomes – one set inherited from each parent. There are 22 pairs of chromosomes called autosomal chromosomes that carry thousands of genes. The 23rd pair of chromosomes are called the sex chromosomes, as they determine the sex of the individual, which is determined by X and Y chromosomes. Individuals that are homozygous have two identical alleles (XX) and are female, and individuals that are heterozygous have two different alleles (XY) and are male. The presence of a Y chromosome triggers the development of a certain set of male characteristics, and its absence results in the development of female characteristics.

A gamete is a specialized sex cell. In males, the gamete is called a spermatozoon (plural = spermatozoa) or sperm, and in females the gamete is called an ovum (plural = ova) or egg. Sperm and ova are haploid cells, meaning they have only one set, or 23 chromosomes—half the number of somatic cells. When a sperm penetrates an ovum during fertilization, the 23 chromosomes in the sperm combine with the 23 chromosomes in the ovum, forming a new diploid cell called a zygote.

As mentioned above, a male’s sex chromosomes are XY. Because a sperm is haploid and only carries one of the sex chromosomes, it will carry either the X or the Y chromosome. The ovum always carries an X chromosome because both of a woman’s sex chromosomes are X chromosomes. If a sperm carrying the X fertilizes an ovum, the child will be a female (XX). If a sperm carrying the Y fertilizes an ovum, the child will be a male (XY). Therefore, the male determines the sex of the child.